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This invention relates to a novel galanin receptor, designated GALR2, to nucleotides encoding it, and to assays which use it.
Although first isolated from porcine intestine, galanin is widely distributed in the central and peripheral nervous system. Galanin in most species is a 29 amino acid peptide with an amidated carboxyl terminus. Human galanin is unique in that it is longer, 30 amino acids, and is not amidated. There is strong conservation of the galanin sequence with the amino terminal fifteen residues being absolutely conserved in all species. Galanin immunoreactivity and binding is abundant in the hypothalamus, the locus coeruleus, the hippocampus and the anterior pituitary, as well as regions of the spinal cord, the pancreas and the gastrointestinal tract.
Like neuropeptide Y (NPY), injection of galanin into the paraventricular nucleus (PVN) of the hypothalamus produces a dose-dependent increase in feeding in satiated rats. While galanin, like norepinephrine, enhances carbohydrate ingestion, some studies have shown that it profoundly increases fat intake. It has been suggested that galanin shifts macronutrient preference from carbohydrate to fat. The same injections that increase feeding reduce energy expenditure and inhibit insulin secretion. There is enhanced galanin expression in the hypothalamus of genetically obese rats compared with their lean littermate controls. Injection of peptide receptor antagonists into the PVN blocks the galanin-specific induction of increased fat intake. Specific galanin antisense oligonucleotides when injected into the PVN produce a specific decrease in galanin expression associated with a decrease in fat ingestion and total caloric intake while hardly affecting either protein or carbohydrate intake. Thus galanin appears to be one potential neurochemical marker related to the behavior of fat ingestion.
Galanin inhibits cholinergic function and impairs working memory in rats. Lesions that destroy cholinergic neurons result in deficits in spatial learning tasks. While locally administered acetylcholine (ACh) reverses some of this deficit, galanin blocks this ACh-mediated improvement. Evidence from autopsy samples from Alzheimer""s disease-afflicted brains suggests an increased galinergic innervation of the nucleus basilis. Thus, if galinergic overactivity contributes to the decline in cognitive performance in Alzheimer""s disease, galanin antagonists may be therapeutically useful in alleviating cognitive impairment.
In the rat, administration of galanin intracerebroventricularly, subcutaneously or intravenously increases plasma growth hormone. Infusion of human galanin into healthy subjects also increases plasma growth hormone and potently enhances the growth hormone response to GHRH.
Galanin levels are particularly high in dorsal root ganglia. Sciatic nerve resection dramatically up-regulates galanin peptide and mRNA levels. Chronic administration of galanin receptor antagonists (M35, M15) after axotomy results in a marked increase in self mutilation behavior in rats, generally considered to be a response to pain. Application of antisense oligonucleotides specific for galanin to the proximal end of a transected sciatic nerve suppressed the increase in galanin peptide levels with a parallel increase in autotomy. Galanin injected intrathecally acts synergistically with morphine to produce analgesia, this antinociceptive effect of morphine is blocked by galanin receptor antagonists. Thus, galanin agonists may have some utility in relieving neural pain.
The actions of galanin are mediated by high affinity galanin receptors that are coupled by pertussis toxin sensitive Gi/Go proteins to inhibition of adenylate cyclase activity, closure of L-type Ca++ channels and opening of ATP-sensitive K+ channels. Specific binding of 125I-galanin (Kd approximately 1 nM) has been demonstrated in areas paralleling localization of galanin immunoreactivity: hypothalamus, ventral hippocampus, basal forebrain, spinal cord, pancreas and pituitary. In most tissues the amino terminus (GAL 1-15) is sufficient for high affinity binding and agonist activity.
Recently, a galanin receptor cDNA was isolated by expression cloning from a human Bowes melanoma cell line. (Habert-Ortoli, et al. 1994. Proc. Nat. Acad. Sci, USA 91: 9780-9783). This receptor, GALR1, is expressed in human fetal brain and small intestine, but little else is known of its distribution. Gal(1-16) is at least 1000 times more active than pGAL(3-29) as an inhibitor of 125I-porcine galanin binding to this receptor transiently expressed in COS cells. It remains to be determined whether this receptor subtype represents the hypothalamic receptor that mediates the galanin specific feeding behavior.
It would be desirable to identify further galanin receptors so that they can be used to further characterize this biological system and to identify galanin receptor subtype selective agonists and antagonists.
This invention relates to a novel galanin receptor, designated GALR2, substantially free from associated proteins, and to GALR2-like receptors which are at least about 40% homologous and which have substantially the same biological activity. In preferred embodiments of this invention, the GALR2-like receptors are at least about 60%, and more preferably at least about 75%, and even more preferably at least about 85% homologous to a GALR2 receptor. This invention also relates specifically to rat, human and mouse GALR2, substantially free from associated proteins, and to receptors which are at least about 50% homologous and which have substantially the same biological activity.
Another aspect of this invention are primate and non-primate GALR2 proteins which are encoded by substantially the same nucleic acid sequences, but which have undergone changes in splicing or other RNA processing-derived modifications or mutagenesis-induced changes, so that the expressed protein has a homologous, but different amino acid sequence from the native forms. These variant forms may have different and/or additional functions in human and animal physiology or in vitro in cell based assays.
A further aspect of this invention are nucleic acids which encode a galanin receptor or a functional equivalent from rat, human, mouse, swine, or other species. These nucleic acids may be free from associated nucleic acids, or they may be isolated or purified. The nucleic acids which encode a receptor of this invention may be any type of nucleic acid. Preferred forms are DNAs, including genomic and cDNA, although this invention specifically includes RNAs as well. Nucleic acid constructs may also contain regions which control transcription and translation such as one or more promoter regions, termination regions, and if desired enhancer regions. The nucleic acids may be inserted into any known vector including plasmids, and used to transfect suitable host cells using techniques generally available to one of ordinary skill in the art.
Another aspect of this invention are vectors comprising nucleic acids which encode GALR2, and host cells which contain these vectors. Still another aspect of this invention is a method of making GALR2 comprising introducing a vector comprising nucleic acids encoding GALR2 into a host cell under culturing conditions.
Yet another aspect of this invention are assays for GALR2 ligands which utilize the receptors and/or nucleic acids of this invention. Preferred assays of this embodiment compare the binding of the putative GALR2 ligand to the binding of galanin to GALR2.